1. Field of the Invention
The power supply sensing circuitry of the present invention is used in conjunction with data processing systems such as but not limited to data processing systems which control the functions of typewriters and other office equipment machines. The present invention is concerned with circuitry for determining whether the power supply is in a brown out or power down condition.
2. Description of the Prior Art
Prior art patents concerned with power line supply sensing circuitry include the Putterman et al patent, U.S. Pat. No. 3,624,617, which discloses a protection circuit for a computer memory having a plurality of power sources 20, 21 and 22, as seen in FIG. 2. The outputs of the power sources are ANDed by gate 28 to control the system. In particular, each power supply includes a detector transistor comprising transistors 58, 78 and 89 in FIG. 4a, which turns ON or OFF depending on whether the mechanical power source is functioning properly. Diodes 101, 102 and 103, and resistor 105 form a conventional AND gate. If one of the power sources fails or is disrupted, the AND gate produces a power failure signal which inhibits various computer operations.
Another such patent is the Pollitt patent, U.S. Pat. No. 3,757,302, which discloses a power failure detection system that automatically monitors the supply voltage to indicate a power fail condition. In FIG. 1, there is shown a primary A.C. power source 10 which is rectified by rectifier 14 and filtered by filter 16. The resulting voltage is applied to one input of a comparator 20, the other input thereto is supplied by a reference voltage. The comparator 20 thereby provides an output whenever the supply voltage is below the minimum acceptable level of the power source 10.
A patent to Graf et al, U.S. Pat. No. 3,800,198, shows an overvoltage and undervoltage prevention system for a motor controller. The voltage from the power supply between points 22 and 24 provides a sensed voltage on line 72 via voltage signal generation 70 to overvoltage detector 74 and undervoltage detector 80. The positive voltage on 72 is compared with two fixed negative voltages 108 and 128 (FIG. 2). If the positive voltage on 72 and appropriate voltage divider is below the voltage set by 108, a positive voltage is provided at 78 and if it is above the voltage set by 128 and appropriate voltage divider, a positive voltage is set at 84. Stop signal generator monitors the sum of these voltages at 86 and provides a positive in-range signal at 92 when both signals are present at 86. If the input at 72 is too high, the output on 78 is negative. If the input at 72 is too low, the output on 84 is ground, for reduction of either of the output signals at 86 will cause stop generator 90 to provide a grounded output. Circuits 74 and 80 are comparator circuits and circuit 90 is an AND gate monitoring the output of these comparator outputs to provide a window detector.
Another prior art patent, U.S. Pat. No. 3,918,043, to Beck is also concerned with power line sensing circuitry. The Beck patent discloses a power supply monitor which senses the loss of the primary power supply signal through voltage detector 34. The threshold voltage is determined by zener diode 32.
Another such patent is the Driscoll et al patent, U.S. Pat. No. 4,245,150, which discloses a power line disturbance detector circuit that includes a comparator 26 for comparing primary AC power to a DC reference voltage provided by a voltage regulator. The AC supply voltage is rectified and filtered by diode 32 and capacitor 34. Resistors 36, 38 form a voltage divider such that the resulting voltage is applied to the non-inverted input of comparator 41. The inverted input of the comparator 41 is the output of voltage regulator 30. The comparator generates a reset pulse once during each AC cycle as long as the AC voltage exceeds the DC reference voltage. If this condition is not met, a power line disturbance signal is generated.
Another related patent is the Genuit et al, U.S. Pat. No. 3,843,907, which discloses an adjustable current detector that monitors the current delivered by each section of a multisection switching regulator. The currents are monitored by comparators 64a and 64b, the outputs of which are ORed together by OR gate 66. If either section of the regulator becomes defective, the output of the OR gate will go high to disable the circuit.
None of the aforementioned patents, however, disclose a power supply sensing circuitry embodying the present invention wherein a first comparator determines whether the power supply potential exceeds by a predetermined amount the minimum voltage required to operate a data processing system connected to the power supply, and a second comparator determines whether the power supply potential is at least the voltage required to operate the connected data processing system.